Manning was not a participant in any of the actions recorded in the documents he disclosed, which date back as far as 1966. Snowden appears to have been involved in the programs he revealed.
"If I wanted to see your emails or your wife's phone, all I have to do is use intercepts," he told the Guardian. "I can get your emails, passwords, phone records, credit cards."
Clark says this difference may be important.
"The moral case for engaging in whistleblowing may be different whether you are a witness to wrongdoing or whether you believe you may have participated in wrongdoing," she said.
Prosecutors say Manning attempted to conceal his leak. He was arrested after a hacker to whom he had confided his actions went to the FBI.
Snowden, in contrast, fled from his workplace in Hawaii to Hong Kong, where he asked the Guardian to reveal his identity — and, in a video interview with the newspaper, began his defense.
"I never heard from Manning anything like the elaborate justification for the actions that Snowden is offering," said Thomas Mockaitis, a professor of history at DePaul University in Chicago. "Snowden is making a pretty clear moral argument.
"I'm not saying I accept it, mind you," he said. "But I'm kind of wondering whether he isn't laying the groundwork for his trial."
Mockaitis wonders what else Manning and Snowden might have in common.
"What may be interesting to consider is whether below the surface you have anything in terms of a similar personality type," he said. "That there was some kind of thrill-seeking. I don't know."
Mockaitis said the rise of the Internet and social media could be fueling a change in perceptions of information and secrecy.
"For our generation," said Mockaitis, who is 58, "information is power and control and needs to be hoarded and managed. For this Facebook generation, information is free."
Kettl, of the University of Maryland, spoke of "the instinct to share everything that one knows with everybody."
"There's a question about whether or not there's a kind of cultural issue that's creeping in with the new generation of workers who have come of age in the Facebook age and therefore would be more inclined to use social networking and other kinds of media to tell everybody whatever it is I think they need to know," he said.
"Two cases are far too few to be able to draw any kind of conclusions, but it's surely something on the minds of the people who run the nation's security system."